This invention is primarily directed to a system and methods for use in offshore oil and gas production, where there is in many cases a need for transferring fluids between a floating installation and a tanker. Thus, the floating installation (FPSO) serves for more or less temporary storage of hydrocarbon fluids being produced, with tankers being employed for the actual export of the products.
Systems for such purposes are known, wherein the floating installation is permanently moored to the seabed, and comprising a buoy being moored to the seabed and connected to the installation through a first mooring line and a first fluid transfer line, and being adapted to be connected to the tanker through a second mooring line and a second fluid transfer line.
A typical example of such a system is found in U.S. Pat. No. 5,065,687, describing a mooring buoy to be located on the sea surface and making possible the weathervaning of a moored tanker vessel through a certain, but limited arc of a circle.
The present invention has for an important object to provide a cost effective, reliable fluid offtake system with high operability, for the purposes indicated above. This is obtained by utilizing the principle of a mooring buoy being immersed under the sea surface, as will be described further below.
At this point it should be made clear that submerged mooring buoys as such are previously known, for example from U.S. Pat. No. 5,816,183. This and other examples, however, are not related to the particular arrangement where a large floating installation constitutes an essential component in the complete mooring and fluid transferring system.
The heart of the present system is the buoy, forming a base unit with buoyancy and having all required facilities. It is partly moored to the sea bottom and partly to the FPSO; thus the whole system can assist in mooring the FPSO. Equilibrium of the buoy is ensured by proper load and load attachment, represented mainly by mooring lines connected to the buoy. Means for mooring a shuttle tanker are provided on top of the buoy. All functions on the base unit or buoy may be controlled via an umbilical cable from the FPSO.
As will be seen from the following description the mooring of the buoy is asymmetric, for example by having four mooring lines directed away from the FPSO and two lines attached to the FPSO.
The system according to this invention, as defined in the claims, involves advantages as follows:
No collision danger, as will be present with surface buoys Significantly reduced risk of collision with the floating installation (FPSO).
Eliminates contribution to tanker hawser tension variation by buoy (negligible) movements.
Easy installation with dry (no diving) connections and installation of main components before immersing the mooring buoy.
Simple export hose arrangement from FPSO, with easy installation, inspection and replacement.
270 degrees normal weathervaning/full 360 degrees capability Eliminating polyester mooring line elongation problems.
As a typical example of dimensions and capacities in a practical embodiment of the system according to the invention, the following is given:
Export tankers (shuttle tankers) for use with the system can be of sizes substantially corresponding to what is indicated above with respect to the FPSO.
Typical buoy floating depth 50-100 m.
Buoy net buoyancy 250-300 tonnes.
From the above example it will be understood that the mooring capabilities and properties of the system are of utmost importance for performing the tanker loading operations under varying conditions of wind and waves. Thus, the system according to the invention is capable of performing well in more severe conditions than most existing systems. A down time requirement of not more than 1% per year throughout 20 years life time, should be satisfied at the location given.